The present invention relates to a membrane with low air permeability, yet having good moisture permeability. This membrane is used for various applications such as heat exchanger membranes, humidifier membranes, dehumidifier membranes, pervaporation membranes (membranes for separating water from other liquids, such as ethanol, for example), and the like.
Heat exchanger membranes are a typical application for membranes. Heat exchanger membranes are utilized, for example, in air conditioning systems, and can allow heat exchange without mixing of inside and outside air. In recent years, there have been proposed total heat exchanger membranes that exchange not just heat but humidity as well. Preferably, such heat exchanger membranes will have low air permeability, yet have good moisture permeability.
In FIG. 1, shows a prior art heat exchanger membrane 10 of double-layer construction, composed of a cured moisture-permeable resin layer 30 disposed on a polymer resin porous sheet 20. Reinforcing the heat exchanger membrane 10 has also been proposed. As shown in FIG. 2, for example, there is shown a heat exchanger membrane 11 of triple-layer construction having an additional reinforcing member 40 disposed on the cured moisture-permeable resin layer 30. These heat exchanger membranes 10, 11 exhibit low air permeability and good moisture permeability, and are also resistant to condensation, loss of shape, growth of mildew, and the like, giving them extended service life. While Ppolyvinyl alcohol subjected to at least partial crosslinking has been proposed as an exemplary moisture-permeable resin, polyurethane resins, silicone resins, fluororesins and the like were actually used as moisture-permeable resin in Working Examples.
As a result, it has been discovered that while the heat exchanger membrane of the prior art afford excellent anti-condensation properties, these anti-condensation properties are diminished when certain moisture-permeable resins (e.g., polyvinyl alcohol, etc.) are used. With the double-layer heat exchanger membrane 10 in particular, it was found that since the moisture-permeable resin layer 30 lies exposed on the surface of the heat exchanger membrane, if the moisture-permeable resin 30 per se has low water resistance, the moisture-permeable resin will tend to become washed away by water droplets forming through condensation. Even where the moisture-permeable resin layer 30 is covered by the reinforcing member 40, as in the triple-layer structure (see heat exchanger membrane 11 in FIG. 2), where certain moisture-permeable resins (e.g., polyvinyl alcohol etc.) are used, the moisture-permeable resin can dissolve, resulting in pinholes forming in the moisture-permeable resin layer 30 or in diminished bonding strength between the moisture-permeable resin layer 30 and the reinforcing member 40, so that anti-condensation properties are not satisfactory. Additionally, where the triple-layer structure is employed, since the polymer resin porous sheet 20 and the reinforcing member 40 are laminated together via the cured moisture-permeable resin layer 30, bonding between the polymer resin porous sheet 20 and the reinforcing member 40 cannot be considered to be good, and delamination sometimes occurs when subjected to external force.